Patch testing is essential both as a prevention and as a solution for skin allergies, but many of us are doing it wrong. One common mistake is patch testing in entirely different areas than where the product is intended to work. Body parts have varying skin thicknesses, and as a result, can show different reactions to products. For skincare formulas, you should patch test on your jawline, neck, or under your ear as these areas are the closest to facial skin. As for how much product you should use, a dime size amount is recommended.